1. Field of the Invention
The present invention relates to a sputtering device used in the fabrication of various types of semiconductor devices. More particularly, it relates to a sputtering device suited to the deposition of films inside holes with a high aspect ratio.
2. Discussion of Related Art
With semiconductor devices, such as various types of memory and logic devices, a sputtering process is used in the production of various wiring films, and in the production of barrier films that prevent the interdiffusion of different layers. A sputtering process uses a sputtering device, and there has recently been a great need for such sputtering devices to allow the inner surfaces of holes formed in a substrate to be coated with a good degree of coverage.
A CMOS-FET (Field Effect Transistor), which is commonly used in a DRAM, employs a structure that prevents cross-contamination between the contact wiring layer and the diffusion layer by the provision of a barrier film to the inner surface of the contact holes provided on the diffusion layer. With a multilayer wiring structure for the wiring of memory cells, through holes are provided to an interlayer insulation film, and interlayer wiring is embedded inside these through holes in order to link the lower layer wiring with the upper layer wiring. Here again, a structure is adopted in which a barrier film is produced inside the through holes to prevent cross-contamination.
Because of the increasing degree of integration, the aspect ratio of these holes (the ratio of the hole depth to the size of the hole opening) has been steadily rising over the years. For example, the aspect ratio is about 4 with a 64 megabit DRAM, and is about 5 to 6 with a 256 megabit DRAM.
In the case of a barrier film, a thin film must be built up on the bottom of the hole in an amount of 10 to 15% of the amount of build-up on the peripheral surfaces of the hole. For holes with a high aspect ratio, it is difficult to deposit a film with a high bottom coverage (the ratio of the deposition rate on the hole bottom to the deposition rate on the peripheral surfaces of the hole). A decrease in the bottom coverage can lead to a thinner barrier film at the bottom of the hole and to critical flaws in the device characteristics, such as junction leakage.
Collimation sputtering and low-pressure long-distance sputtering processes have been developed up to now as sputtering processes that increase the bottom coverage. Collimation sputtering involves using a plate (collimator) in which numerous holes have been made in the direction perpendicular to the substrate, and providing this plate between the target and the substrate. Having the sputter particles pass through these holes selectively causes only those sputter particles that fly more or less perpendicular to the substrate to arrive at the substrate.
Low-pressure long-distance sputtering involves lengthening the distance between the target and substrate (usually about 3 to 5 times farther) so that relatively more of the sputter particles that fly more or less perpendicular to the substrate will land on the substrate, and lowering the pressure more than usual (about 0.8 mTorr or less) so that the free mean path is longer, which results in less turbulence of these sputter particles.
Collimation sputtering yields better results than an ordinary sputtering device in that a bottom coverage of about 20 to 30% is obtained with respect to a hole with an aspect ratio of 2. However, a problem with collimation sputtering is that sputter particles accumulate on the collimator portion, and the resulting loss of material decreases the film deposition rate, or the thin film that builds up on the collimator can peel off and become dust particles. Collimation sputtering is held to be limited to devices of the 16-megabit class, in which the aspect ratio is about 3.
With low-pressure, long-distance sputtering, a bottom coverage of approximately 40% is obtained for a hole with an aspect ratio of 2, and a bottom ratio of approximately 20% for a hole with an aspect ratio of 3.5, so the performance is better than with collimation sputtering. However, since the pressure is lowered and the distance between the target and the substrate is lengthened with low-pressure long-distance sputtering, there is a fundamental decrease in the film deposition rate. Therefore, low-pressure long-distance sputtering is also held to be limited to devices with an aspect ratio of about 4.